程序性能优化---倒置字符串中单词

需求：

输入 have a nice day

输出 day nice a have

实现：

#include <unistd.h>
#include <iostream>
#include <vector>
#include <sstream>
#include <stack>
using namespace std;
string str = "Have a nice day";
void fun(string &res) {
    stack<string>ss;
    stringstream os(str);
    string word;
    while (os >> word) {
        ss.push(word);
    }
    while (false == ss.empty()) {
        res += ss.top();
        res += " ";
        ss.pop();
    }
}
void fun1(string &res) {
    stack<string>ss;
    stringstream os(str);
    string word;
    while (os >> word) {
        ss.push(word);
    }
    static string blank = " ";
    while (false == ss.empty()) {
        res += ss.top();
        res += blank;
        ss.pop();
    }
}
void fun2(string &res) {
    string word;
    stack<string>ss;
    for (auto &ch : str) {
        if (' ' == ch)  {
            ss.push(word);
            word.clear();
            continue;
        }
        word += ch;
    }
    ss.push(word);
    static string blank = " ";
    while (false == ss.empty()) {
        res += ss.top();
        res += blank;
        ss.pop();
    }
}
void fun3(string &res) {
    char word[64] = "";
    int index = 0;
    int size = str.size();
    for(int i = size - 1;i >= 0;i--) {
        if (' ' != str[i]) {
            word[index++] = str[i];
        }
        else {
            for (int i = index - 1;i >= 0;i--) {
                res += word[i];
            }
            res += str[i];
            index = 0;
        }
    }
    for (int i = index - 1;i >= 0;i--) {
        res += word[i];
    }
}


int main() {
    string res;
    for (int i = 0;i < 1000000;i++) {
        //fun(res);         // 2.05s
        //fun1(res);        // 1.970s
        //fun2(res);        // 1.875s
        fun3(res);          // 0.675s
        res.clear();
    }

    return 0;
}

make.sh

g++ -std=c++17 -g -o Test test.cpp -pthread